Light scattering and conversion plate for leds

ABSTRACT

The invention relates to an illumination system having a light scattering and conversion plate comprising a non-converting but scattering layer and a thinner converting layer. By separating scattering and conversion, the characteristics of the illumination system can greatly be increased.

CROSS REFERENCE TO PRIOR APPLICATIONS

This application is a Continuation application of U.S. application Ser.No. 13/320,803, filed on Nov. 16, 2011, which is the U.S. National PhaseApplication of International Application No. PCT/IB2010/052167, filed onMay 17, 2010 and claims the benefit of European Patent Application No.09160613.7, filed on May 19, 2009. These applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present invention is directed to novel luminescent materials andcompounds for light emitting devices, especially to the field of LEDs

BACKGROUND OF THE INVENTION

In recent years, several new techniques and setups have been developedfor LEDs, amongst them the introduction of ceramic converter plates andlayers. In this regard, reference is made e.g. to the US 2004/145 308which incorporated by reference.

However, there is still the continuing need for converter plates andlayers which show good emitting and scattering properties.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an illuminationsystem which is usable within a wide range of applications andespecially allows the fabrication of warm white pcLEDs with optimizedluminous efficiency and color rendering.

This object is solved by an illumination system according to claim 1 ofthe present invention. Accordingly, an illumination system is proposedcomprising at least one light scattering and conversion plate comprising

-   -   a) a first layer having scattering properties and no conversion        properties and    -   b) a second layer having conversion properties

whereby the thickness A of the first layer and the thickness B of thesecond layer match

A≧3*B

Preferably A and B match A≧4*B, more preferred A≧7*B.

The term “layer” and/or “plate” especially mean and/or include an objectwhich extends in one dimension (i.e. the height) to ≦40%, more preferred≦20% and most preferred ≦10% than in any of the other dimensions (i.e.width and length).

The term “scattering” especially means and/or includes the change of thepropagation direction of light.

The term “converting” especially means and/or includes the physicalprocess of absorption of light and emitting of light in anotherwavelength area, e.g. due to radiative transitions that involve at leastone ground state and at least one excited state and that may bedescribed with a configurational coordinate diagram showing thepotential energy curves of absorbing and emitting centers as a functionof the configurational coordinate.

The term “no conversion properties” especially means and/or includesthat ≧95%, more preferred ≧97%, more preferred ≧98% and most preferred≧99% of all transmitted light passes the plate without being converted.

Such an illumination system has shown for a wide range of applicationswithin the present invention to have at least one of the followingadvantages:

-   -   The surprising result of one of the basic ideas underlying the        present invention, i.e. separating scattering and converting is        that for most application both a good forward emission together        with an angular stability of the emission profile can be found.    -   The overall setup of the illumination system can be kept simple        and small    -   The lifetime of the illumination system is considerably extended        because of improved heat dissipation properties and improved        chemical stability    -   additional functional layers can be applied on one layer only        eliminating the risk of damaging the more expensive other layer.

According to a preferred embodiment, the second layer is in the opticalpath in between the primary light source and the first layer.

The primary light source will in most applications be a blue LED;however, any devices known in the field to the skilled person in the artmay be used.

According to a preferred embodiment, the first and/or second layer areessentially made out of a ceramic material.

The term “essentially” in the sense of the present invention especiallymeans≧90 (wt.)-%, more preferably ≧95 (wt.)-%, yet more preferably ≧98(wt.)-% and most preferred ≧99 (wt.)-%.

The term “ceramic material” in the sense of the present invention meansand/or includes especially a crystalline or polycrystalline compactmaterial or composite material with a controlled amount of pores orwhich is pore free.

The term “polycrystalline material” in the sense of the presentinvention means and/or includes especially a material with a volumedensity larger than 90 percent of the main constituent, consisting ofmore than 80 percent of single crystal domains, with each domain beinglarger than 0.5 μm in diameter and having different crystallographicorientations. The single crystal domains may be connected by amorphousor glassy material or by additional crystalline constituents.

According to a preferred embodiment, the ceramic material has a volumeof ≧0.005 mm³ to ≦8 mm³, more preferred ≧0.03 mm³ to ≦1 mm³ and mostpreferred ≧0.08 mm³ to ≦0.18 mm³.

According to a preferred embodiment, the ceramic material has a densityof ≧90% and ≦100% of the theoretical density. This has been shown to beadvantageous for a wide range of applications within the presentinvention since then the luminescent properties of the at least oneceramic material may be increased.

More preferably the ceramic material has a density of ≧97%, and ≦100% ofthe theoretical density, yet more preferred ≧98% and ≦100%, even morepreferred ≧98.5% and ≦100% and most preferred ≧99.0% and ≦100%.

According to a preferred embodiment of the present invention, thesurface roughness RMS (disruption of the planarity of a surface;measured as the geometric mean of the difference between highest anddeepest surface features) of the surface(s) of the ceramic material is≧0.001 μm and ≦100 μm.

According to an embodiment of the present invention, the surfaceroughness of the surface(s) of the at least one ceramic material is≧0.005 μm and ≦0.8 μm, according to an embodiment of the presentinvention ≧0.01 μm and ≦0.5 μm, according to an embodiment of thepresent invention ≧0.02 μm and ≦0.2 μm and according to an embodiment ofthe present invention ≧0.03 μm and ≦0.15 μm.

According to a preferred embodiment of the present invention, thespecific surface area of the ceramic material is ≧10⁻⁷ m²/g and ≦0.1m²/g.

According to a preferred embodiment, the thickness B of the second layeris ≧5 μm and ≦80 μm. This has been shown to be advantageous for manyapplications within the present invention since by doing so the packingefficiency and side emission of the light scattering and conversionplate may greatly be increased and reduced, respectively.

Preferably the thickness B of the second layer is ≧10 μm and ≦50 μm.

According to a preferred embodiment, the thickness A of the first layeris ≧50 μm and ≦1000 μm. This has been shown to be advantageous, since bydoing so, for many application the scattering features and theuniformity of the light emitting profile of the light scattering andconversion plate may greatly be increased.

Preferably the thickness A of the first layer is ≧100 μm and ≦300 μm.

According to a preferred embodiment, the scattering coefficient of thefirst layer is >0 and ≦1000 cm⁻¹.

The scattering coefficient s is determined by measurement of thereflectance R₀ and/or the transmittance T₀ of a thin layer withthickness A according to the equations:

${sA} = {\frac{1}{b}{{Arctgh}( \frac{1 - {kR}_{0}}{{bR}_{0}} )}}$${sA} = {\frac{1}{b}( {{{Arsinh}\frac{b}{T_{0}}} - {{Arsinh}\mspace{14mu} b}} )}$With $b = \sqrt{k^{2} - 1}$ and $k = \frac{s + a}{s}$

(a is the absorption coefficient of the layer)

In case of a=0 equations simplify to

${sA} = \frac{R_{0}}{1 - R_{0}}$ ${sA} = \frac{1 - T_{0}}{T_{0}}$

Preferably the scattering coefficient of the first layer is ≧100 and≦500 cm⁻¹.

According to a preferred embodiment, the mean refractive index n of thefirst layer is ≧1.3 and ≦2.5.

According to a preferred embodiment, the difference Δ_(n) in refractiveindex between the refractive index of the first layer and the secondlayer is ≧0.03 and ≦1. This has been shown to be advantageous for mixingthe light from the primary light source and the converted light from thesecond layer for many applications. Preferably Δ_(n) is ≧0.3 and ≦0.5.

According to a preferred embodiment, the first layer is essentially madeout of a material selected from the group comprising glass, Al₂O₃,Y₃Al₅O₁₂, RE₃Al₅O₁₂ (RE=rare earth metal), Y₂O₃, ZnS, AlON, AlPON, AlN,MgAl₂O₄, SiC, SiO₂, Si₃N₄ or mixtures thereof.

According to a preferred embodiment, the second layer is essentiallymade out of a material selected from the group comprisingLu_(b)Y_(c)Gd_(d)Ce_(e)Al₅O₁₂ with b+c+d+e=3, 0.09≦e≦0.24,Ca_(1+x+y+z+0.5u)M_(x)Si_(1+u+v+z)Al_(1−u+v+z)N_(3−v)O_(v):Eu_(y),Ce_(z) with 0≦u<0.2, 0<v<0.05, 0≦x<1, 0.001≦y≦0.01, 0.002≦y≦0.04, andM=Sr, Ba, Mg or mixtures thereof.

According to a preferred embodiment, the plate comprises a third layerprovided in between the first and second layer and essentially made outof an adhesive material, preferably a silicone glue.

This invention furthermore relates to method of producing a lightscattering and conversion plate comprising the steps of

-   a) Providing a first and second layer, whereby the thickness of the    second layer is larger than desired-   b) Connecting the first and second layer, optionally by use of a    third layer provided in between the first and second layer and    essentially made out of an adhesive material-   c) Mechanically reducing the thickness of the second layer, e.g. by    grinding

By doing so, it has been shown for many applications that a lightscattering and conversion plate according to the present invention canbe made easily and effectfully even for small thicknesses of the secondlayer.

An illumination system according to the present invention may be of usein a broad variety of systems and/or applications, amongst them one ormore of the following:

-   -   Office lighting systems    -   household application systems    -   shop lighting systems,    -   home lighting systems,    -   accent lighting systems,    -   spot lighting systems,    -   theater lighting systems,    -   fiber-optics application systems,    -   projection systems,    -   self-lit display systems,    -   pixelated display systems,    -   segmented display systems,    -   warning sign systems,    -   medical lighting application systems,    -   indicator sign systems, and    -   decorative lighting systems    -   portable systems    -   automotive applications    -   green house lighting systems

The aforementioned components, as well as the claimed components and thecomponents to be used in accordance with the invention in the describedembodiments, are not subject to any special exceptions with respect totheir size, shape, material selection and technical concept such thatthe selection criteria known in the pertinent field can be appliedwithout limitations.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional details, features, characteristics and advantages of theobject of the invention are disclosed in the subclaims, the figures andthe following description of the respective figures and examples,which—in an exemplary fashion—show several embodiments and examples of aluminescent material for use in an illumination system according to theinvention as well as several embodiments and examples of an illuminationsystem according to the invention.

FIG. 1 shows a very schematic setup of an illumination system accordingto one embodiment of the present invention.

FIG. 2 shows a detailed very schematic view of the light scattering andconverting plate of FIG. 1

FIG. 3 shows a photograph of a detail of a light scattering andconverting plate according to Example I of the present invention.

FIG. 4 shows a photograph of a detail of a light scattering andconverting plate according to Example II of the present invention.

FIG. 5 shows the spatial radiation pattern of two LEDs according toExample I and II of the present invention and a comparative example

FIG. 6 shows the Emission spectra of the two LEDs according to Example Iand II of the present invention and the comparative example; and

FIG. 7 shows a CIE 1931 Chromaticity chart showing the Planckian locusof the two LEDs according to Example I and II of the present inventionand the comparative example

FIG. 1 shows a very schematic setup of an illumination system accordingto one embodiment of the present invention. Most of FIG. 1 is prior artand known to the skilled person and will therefore described onlybriefly. It is apparent that the illumination system of FIG. 1 isexemplarily only and the skilled person may use different parts orreplace it at lib.

The illumination system 1 comprises a thin film chip blue LED 20 uponwhich a light scattering and converting plate 10 is provided; both arecovered by a lens.

FIG. 2 shows a detailed very schematic view of the light scattering andconverting plate of FIG. 1. The plate 10 comprises a first layer 12(with a thickness A), a second layer 14 (with a thickness B) and inbetween a silicon glue layer 16. FIG. 2 is highly schematic and in mostapplications the actual dimensions will be much different. It is notedthat the plate 10 is provided in the illumination system 1 so that thesecond layer 12 is in the optical path between the blue chip LED 20 andthe second layer 14.

The invention will be further understood by the following Examples I andII which—in a merely illustrative fashion—shows several illuminationsystems of the present invention.

In the examples a Y_(2.88)Ce_(0.012)Al₅O₁₂ layer (i.e. the second layer14) was ground from both sides from a 1.1 mm thick wafer after sinteringto 300 μm thickness. Also a 1 mm thick polycrystalline Al₂O₃ layer (PCA,the first layer 12) with a mass density of 99.98 percent of thecrystalline Al₂O₃ was ground to 150 μm. Then the first layer was coatedwith a Silicone layer (Shin Etsu KJR-9222A and KJR-9222A, mixing ratio1:1), the second layer was attached and the silicone layer was hardenedat a temperature of 100° C. for one hour and cured at 150° C. for 2hours.

After the gluing, the second layer 14 was further ground to a thicknessof 17 (Example I) and 30 μm (Example II), respectively.

Then both plates were diced to 0.99×0.99 mm², mount on a blue TFFC LEDemitting at about 450 nm and lensed. LED packaging was done in the samewas as for Lumiramic phosphor converted LEDs.

Furthermore a comparative Example I was made in analogy to the inventiveExamples. In this comparative example, the second layer was set to athickness of 120 μm and made out of Y_(2.842)Gd_(0.15)Ce_(0.008)Al₅O₁₂(i,e. a lower Cerium-Content to match the higher thickness)

FIG. 3 shows a photograph of a detail of a light scattering andconverting plate according to Example I of the present invention. Thelight scattering and converting plate has an overall thickness of about205 μm, whereby the first layer is about 140 μm thick, the second layerabout 30 μm and the silicon layer about 35 μm.

FIG. 4 shows a photograph of a detail of a light scattering andconverting plate according to Example II of the present invention. Bychance, the light scattering and converting plate has also an overallthickness of about 205 μm; however, here the first layer is about 146 μmthick, the second layer about 17 μm and the silicon layer about 42 μm.

FIG. 5 shows the spatial radiation pattern of the two LEDs according toExample I and II of the present invention and a comparative example. Dueto emission from the faces of the Lumiramic converter plate of thecomparative example the radiative flux under large emission angle ishigher compared to the flux under large angles emitted by LEDs withconverter plates of Example I and Example II of the present invention.

FIG. 6 shows the Emission spectra of the two LEDs according to Example Iand II of the present invention and the comparative example. For thecomparative example (120 μm) the total conversion strength is low andthe peak of the emitted blue light is much larger than the maximum ofthe converted light. For the 30 μm Example both peaks are about equal,which shows the high efficacy of the light scattering and conversionplate.

FIG. 7 shows a CIE 1931 Chromaticity chart showing the Planckian locusand the color points of the two LEDs according to the Example I and IIof the present invention and the comparative example. It can be seenthat due to the inventive scattering and light emitting plate a lowcolor temperature of about 4000K has been realized with the 30 μmExample, whereas only a small change in size of the conversion layer 12(i.e. to 17 μm) leads to a dramatic increase of color temperature toabout 10000K. Variation of thickness and Cerium concentration givesaccess to white LED production of a color temperature ranging from above10 000K to 4000K and below.

The particular combinations of elements and features in the abovedetailed embodiments are exemplary only; the interchanging andsubstitution of these teachings with other teachings in this and thepatents/applications incorporated by reference are also expresslycontemplated. As those skilled in the art will recognize, variations,modifications, and other implementations of what is described herein canoccur to those of ordinary skill in the art without departing from thespirit and the scope of the invention as claimed. Accordingly, theforegoing description is by way of example only and is not intended aslimiting. In the claims, the word “comprising” does not exclude otherelements or steps, and the indefinite article “a” or “an” does notexclude a plurality. The mere fact that certain measures are recited inmutually different dependent claims does not indicate that a combinationof these measures cannot be used to advantage. The invention's scope isdefined in the following claims and the equivalents thereto.Furthermore, reference signs used in the description and claims do notlimit the scope of the invention as claimed.

1. An illumination system, comprising at least one light scattering andconversion plate comprising: a first layer having scattering propertiesand essentially no conversion properties and having a thickness A; and asecond layer having conversion properties and having a thickness B,wherein the thickness A of the first layer is greater or equal to fourtimes the thickness B of the second layer.
 2. The illumination systemaccording to claim 1, wherein the thickness A of the first layer isgreater or equal to seven times the thickness B of the second layer. 3.The illumination system according to claim 1, wherein the second layeris in the optical path in between the primary light source and the firstlayer.
 4. The illumination system according to claim 1, wherein thefirst layer includes a parallelepiped member having a height, a widthand a length, the height being less than or equal to 40 percent of thewidth and less than or equal to 40 percent of the length.
 5. Theillumination system according to claim 1, wherein the first layerincludes a parallelepiped member having a height, a width and a length,the height being less than or equal to 10 percent of the width and lessthan or equal to 10 percent of the length.
 6. The illumination systemaccording to claim herein the thickness B of the second layer is ≧5 μmand ≦80 μm.
 7. The illumination system according to claim 1, wherein thethickness B of the second layer is ≧10 μm and <50 μm.
 8. Theillumination system according to claim 1, wherein the thickness A of thefirst layer is ≧50 μm and ≦1000 μm
 9. The illumination system accordingto claim 1, wherein the thickness A of the first layer is ≧100 μm and≦300 μm
 10. The illumination system according to claim 1, wherein thescattering coefficient of the first layer is >0 and ≦1000 cm⁻¹.
 11. Theillumination system according to claim 1 wherein the difference Δ_(n) inrefractive index between the refractive index of the first layer and thesecond layer is ≧0.03 and ≦1
 12. The illumination system of claim 1wherein the plate comprises a third layer disposed in between the firstand second layer, wherein the third layer is comprised of an adhesivematerial.
 13. The illumination system according to claim 1, wherein afirst layer permits less than 95 percent of light to pass the layerwithout being converted.
 14. The illumination system according to claim1, wherein the first and/or second layer are essentially made out of aceramic polycrystalline material.
 15. The illumination system accordingto claim 14, wherein the ceramic has a density greater or equal to 90percent and less than or equal to 100 percent of the theoreticaldensity.
 16. The illumination system according to claim 14, wherein theceramic has a volume density larger than 90 percent of the mainconstituent.
 17. The illumination system according to claim 14, whereinthe ceramic consists of more than 80 percent single crystal domains,each of the domains being larger than 0.5 μm in diameter.
 18. Theillumination system according to claim 17, wherein at least one domaindoes not have the same crystallographic orientations as another domain.19. The illumination system according to claim 14, wherein the Root MeanSquare surface roughness of the ceramic is greater or equal to 0.001 μmand less than or equal to 1 μm.
 20. The illumination system of claim 2wherein the plate comprises a third layer disposed in between the firstand second layer, wherein the third layer is comprised of an adhesivematerial.